The immunologic characterization of the leukemic cell should facilitate investigations designed to develop an understanding of the heterogeneity and pathophysiology of acute leukemias. The intention here is to continue studies characterizing leukemic cells, their heterogeneity and state of differentiation with heteroantibodies, antibodies and monoclonal antibodies defined by somatic cell hybridization techniques. The identification of these unique antigens by serologic methods is the first step in their isolation for biochemical characterization. It is believed that these studies will provide insight into the nature of the leukemic process and the relationship among these antigens and those found during normal cellular differentiation. These antibodies including T, T3, T4, T5, T6, T9, T10, Ia, J-5 have already allowed the initial classification of T and Null cell leukemias as well as their relation to normal differentiation.